For a variety of reasons, it is often necessary to detect the presence and absence of an occupant in the seat of a vehicle. For example, in the case of an industrial vehicle such as a lift truck, it is common practice to disable the lift truck operating system except when an operator is sitting in the lift truck seat.
Various methods of sensing the presence and absence of an occupant in the seat of a vehicle have been proposed in the past. In the simplest designs, a mechanical switch is embedded in the seat cushion, or is otherwise mechanically linked to motion of the seat. The mechanical switch produces a signal in response to a predetermined amount of weight placed on the vehicle seat. In a more complex arrangement, U.S. Pat. No. 3,764,861, filed on July 31, 1972, and issued to Stephen J. Orris, discloses an electronic detection device. A capacitive switch is placed within the cushion of a vehicle seat, and produces a signal in response to a sensed shift in capacitance caused by the presence of an object on the vehicle seat.
Although various seat switches, such as those described above, have been proposed in the past, none have been found to be fully satisfactory. Such prior seat switches are subject to damage, especially owing to repetitive flexing of the sensing elements caused by the weight of the occupant of the vehicle seat, and exacerbated by vibration of the vehicle. In the case of electronic sensors, many are excessively complex and subject to failure when operated in an industrial environment. Additionally, most sensing devices located within or attached to the vehicle seat can be inadvertently operated by placing an object on the seat which is of sufficient mass to trigger the sensing mechanism.
The present invention is directed to overcoming one or more of the problems as set forth above.